Heavy out of frustration

Researchers at RIKEN’s Discovery Research Institute in Wako, in collaboration with colleagues at Nagoya University, have unraveled the mechanism that leads to an unusual electronic state in the metal compound LiV₂O₄.

At low temperatures, certain metallic materials containing magnetic elements sometimes show a peculiar behavior, where the propagating electrons have an extraordinarily heavy mass: about 100 to 1000 times larger than that of ‘real’ electrons. Known as heavy fermions, these heavy electrons are believed to originate from an interaction between the propagating electrons and the magnetic elements.

Dubbed ‘Kondo coupling’, this intriguing interaction was first described in 1964 by the Japanese physicist Jun Kondo. In Kondo coupling, the heavy fermions are comprised of electrons that are slowed down by the surrounding magnetic elements (Fig. 1a). Recently, however, a few materials—including LiV₂O₄—have been discovered to show heavy fermion properties, but none of the tell-tale signs of Kondo coupling.

Led by Hidenori Takagi from RIKEN, the researchers have investigated the origin of the heavy fermions in LiV₂O₄. The existence of heavy fermions in the absence of Kondo coupling suggests an interesting interaction between electrons and the magnetic elements. Indeed, “based on optical experiments, we are proposing a new mechanism for the heavy fermion mass in LiV₂O₄,” explains Takagi. Their results are published in Physical Review Letters¹.

The researchers studied the optical response of LiV₂O₄ at different wavelengths. The measurements reveal large differences between the material’s reflectivity at room temperature and at low temperatures, where the heavy fermions are formed.

The observed changes in optical response are symptomatic of major changes in the energy states of the material that are not typically a sign of heavy fermions. Rather, they suggest a periodic arrangement on the crystal lattice of the two types of vanadium ions, V³⁺ and V⁴⁺, in LiV₂O₄. Such a periodic arrangement would prevent the formation of heavy fermions. However, the crystal structure of LiV₂O₄ makes any periodic arrangement impossible owing to geometric constraints—it has a ‘geometrically frustrated’ configuration (Fig. 1b,c). The only remaining explanation, then, is that the changes in energy states lead to the formation of heavy fermions.

These results are an important advance, as “this is a new route to heavy fermion formation, and we identified a new electronic state produced by geometrical frustration,” comments Takagi. Further studies might therefore uncover a number of related exotic effects associated with this unusual and complex electronic state.